It's about the various errors displayed by phpBB, like:

"General Error
SQL ERROR [ mysql4 ]

Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2) [2002]

An sql error occurred while fetching this page. Please contact an administrator if this problem persists."

I think these should be translated in all available languages, because most users encounter these errors and they can not solve them because they do not understand them at all (I refer to the non-English users).